Finding a direction of time in exotic particle transformations

Entangled B mesons show that time has an arrow on the smallest scales.

Unlike our daily experience, the world of elementary particle physics is mostly symmetrical in time. Run the clock backward on your day and it won't work; run the clock backward on a process in particle physics and things are just fine. However, to preserve certain fundamental aspects of space-time the Standard Model predicts that certain reversible events nevertheless have different probabilities, depending on which way they go. This time-reversal asymmetry is remarkably hard to observe in practice since it involves measurements of highly unstable particles.

New results from the BaBar detector at the Stanford Linear Accelerator Center (SLAC) have uncovered this asymmetry in time. Researchers measured transformations of entangled pairs of particles, including the rates at which these transformations occurred. Through analyzing over 10 years of data, they found clear time-reversal asymmetry with an error of only one part in 1043, a clear discovery by any standard. These results are a strong confirmation of predictions of the Standard Model, filling in one of the final missing details of that theory.

A direct consequence of relativity in particle physics is the presence of three related symmetries, known as CPT: charge, parity, and time. Charge symmetry (C) involves operations wherein particles are swapped with their antiparticles; parity (P) deals with interactions that may depend on which direction in space they take place. Time reversal (T) is perhaps the most subtle of the three: some processes are predicted to occur differently depending on their order in time. While our everyday lives demonstrate that time has a definite direction (we get older and more decrepit, a tragically dropped pizza will not spontaneously reform itself and become edible again, etc.), fundamental processes involving particles are almost all reversible.

Bottom physics

Mesons are particles consisting of a quark and an antiquark. The B mesons of various sorts all contain a bottom antiquark. Bottom quarks are the second most massive quark, and are highly unstable. As a result, the B mesons quickly transform into other particles. The different types of B particles are determined by the second quark they contain: B0 has a down quark, for example. The corresponding antiparticles are denoted B, and contain bottom quarks. Generally speaking, I won't distinguish between the mesons and anti-mesons, referring to both as B mesons for simplicity.

Additionally, some types of B mesons can oscillate into different types, a process similar to what we see with neutrinos. This oscillation was the key to measuring time-reversal asymmetry in the new BaBar results.

Together, CPT appears to be a true symmetry of nature, to the best of our ability to test it. However, some interactions involving the weak force (one of the four fundamental forces, along with the strong force, electromagnetism, and gravity) violate the CP symmetries. That means that to preserve CPT, these interactions must also violate T—but that has proven remarkably difficult to demonstrate experimentally.

The key to finding T-violating processes is finding particles that exchange identities through oscillations that can go either way. (This is in contrast to the decay of B mesons, which is irreversible.) The new BaBar results examined transitions of different types of B mesons that can transform into other types, and measured the probability of each process happening. To cite one example: the particle B- can change into B0, and vice versa. If T symmetry is violated, then the probability of a B- changing into a B0 will be different from the reverse process.

The BaBar team performed measurements on 468 million pairs of B and B mesons, produced in the decay of Υ(4S) (upsilon) mesons. They observed T-violation in four different processes and their reverses:

B- → B0

B0 → B-

B+ → B0

B0 → B+

(The mesons denoted with a subscript "+" or "-" refer not to the quark makeup, but to the particular way they decay.) These processes were already known to violate CP, so they were prime candidates for looking for T-violation as well. Additionally, each decay of a Υ(4S) produced a BB pair that, by virtue of their common origin, were entangled: measurement of the spin state of one meson revealed the outcome of measurement of its partner.

By identifying the particular B type (its "flavor" in particle physics parlance) and determining the decay process for each pair produced, the researchers measured the rate of transition of one B meson type to another. Since each pair was entangled upon production, the second meson was necessarily in a complementary state at all times, enabling precision measurement of the different transition routes enumerated above. Entanglement is usually the domain of quantum information and measurement theory, making its application to the T-reversal problem exciting and unique.

Thanks to over 10 years of data, the BaBar team measured T-violation to the 14 sigma level, meaning there is only 1 chance in 1043 that this effect is not real. (For comparison, a positive detection of the Higgs boson last summer was announced at the 5 sigma level.) These results are a strong verification of the predictions of the Standard Model of particles, demonstrating that—at least for some elementary particle processes—the direction of time matters.

54 Reader Comments

So, if I'm understanding correctly, time travel is still impossible. And, if you WERE to move "backwards" in time, you wouldn't enter the past but an alternate past because quantum physics would reach alternate results.

So, if I'm understanding correctly, time travel is still impossible. And, if you WERE to move "backwards" in time, you wouldn't enter the past but an alternate past because quantum physics would reach alternate results.

I'm pretty sure time travel is still possible, but you can't do it willy-nilly. For example, if you create a wormhole that connects two points in time, one of those points has to be the moment of creation, and the other has to be in the future. In other words, you'd never be able to go back in time to a point earlier than the moment of creation of the time machine.

But that whole discussion is a different subject than what's presented in the article (I think).

I've wondered if our linear concept of time is partly a social construct and some events actually move backwards, where the effect comes before the cause. If we started looking in reverse, would we see present events that are predictive of past phenomena? Hmmm...

So is it these tiny CPT violations that work as a ratchet to create the time's arrow we perceive in the macroscopic Universe?

Entropy would do that whether CP (or equivalently, T) was violated or not, in a probabilistic sense and as an emergent effect. Meaning, the arrow of time would be discernible with some probability in collections of particles. T violation is interesting because it points to a quantum mechanical foundation for an arrow of time, distinct from the statistical one, that is not an emergent effect.

We've known this was implied by CPT since the discovery of CP violation decades ago. This direct detection of T violation is interesting only for directly confirming CPT is invariant, at least in this system. Finding otherwise would be the death of not just the Standard Model, but Quantum Field Theory in general, which would make any discrepancy in the Higgs data seem mundane by comparison.

Ok so I am unclear with something in this article. Are they saying they saw time going backwards, or are they saying that they assume time is going forwards and that accounts for the asymmetry they saw?

The arrow of time doesn't seem to have to do with the impossibilities of time travel.

You need a local direction of time to have any physics (processes) at all. But in order to build interesting structures (large universes) you want the direction of time to be global.

The absence of time travel prevents physics from being too simple. (Analogous to see that "we have no time travel computers to solve all problems before yesterday, bummer".) So that observation starts where the global direction of time comes in, you want it in order to build structures.

mkuch90 wrote:

And, if you WERE to move "backwards" in time, you wouldn't enter the past but an alternate past because quantum physics would reach alternate results.

The effects of the entropic arrow of time is much larger though, they show up immediately on the coarse grained scale.

On that scale you also have the decoherence arrow of time, having quantum physics making classical systems. You want that arrow too in order for structure formation to happen.

And in turn both the entropic and decoherence arrows who neither is confined to the Standard Model particles, seems to depend on the environmental sink that cosmological expansion provides.

So, unless I am mistaken, it is the cosmological arrow of time that is the major player.

It is interesting to ponder what would cause the SM arrow. Is it the microphysics of the particle fields or the macrophysics of cosmology?

In some TOE it would be the former, I assume. Not built into quantum mechanics itself, but allied with how it implements particle fields.

In eternal inflation it would be the latter, as the SM particle sector happened to freeze out with slightly broken CP & T symmetries in our universe. Then the underlying arrow of time would be the cosmological arrow.

I used to think that the cosmological arrow of time of universes must be caused by them having to expand. And in eternal inflation that requirement becomes that they have to expand in order to be habitable (long lived).

Not so. In Susskind's simple ("tree level") eternal inflation, it is the existence of terminal vacuums (universes with lowest possible vacuum energy) that makes the arrow for the whole process. They act as one way sinks for the process. [ "Fractal-Flows and Time's Arrow", Susskind, arxiv 1203.6440.]

Ok so I am unclear with something in this article. Are they saying they saw time going backwards, or are they saying that they assume time is going forwards and that accounts for the asymmetry they saw?

We can't make time go backwards. They make observations where time goes forward, and when you take CP asymmetry under consideration in processes where you can see T asymmetry, you see T asymmetry. Or at least, that is my understanding.

I'm curious if the problem lies with our perception and understanding of time? The concept of different time directions is intriguing and I hope this leads to furthur research.

I apologize as this is off topic, but I've been stuck on the idea that we do not properly understand the phenomenon of time for a while.. mainly because time zero does not make sense (the best explanation I've gotten is the strength of gravity was so great it effectively paused time, but that still doesn't deliver a solution to the before/where). Hopefully someone here is smarter than I and maybe has at least a hypothesis that I can live with or maybe somewhere in my lifetime someone will come up with one as my brain is not large enough to do so.

As for the time travel thoughts, I've always felt forward travel could be possible, backwards impossible (but viewable) based on our current concepts. Though I'm not a physicist and don't pretend to be, so take that with a grain of salt.

Finding an arrow of time means you can not spontaneously go backward in time while everyone else is going forward. However, the above described effect is so small and so esoteric, it might as well not exist.

So is it these tiny CPT violations that work as a ratchet to create the time's arrow we perceive in the macroscopic Universe?

Entropy would do that whether CP (or equivalently, T) was violated or not, in a probabilistic sense and as an emergent effect. Meaning, the arrow of time would be discernible with some probability in collections of particles. T violation is interesting because it points to a quantum mechanical foundation for an arrow of time, distinct from the statistical one, that is not an emergent effect.

We've known this was implied by CPT since the discovery of CP violation decades ago. This direct detection of T violation is interesting only for directly confirming CPT is invariant, at least in this system. Finding otherwise would be the death of not just the Standard Model, but Quantum Field Theory in general, which would make any discrepancy in the Higgs data seem mundane by comparison.

Actually, I think you have the cart before the horse. The $64k question, which is the one he is really asking, is how does the irreversible macroscopic arrow of time emerge from the completely reversible microscopic laws, which is still an open question. The second law of thermodynamics (net entropy does not decrease) cannot be the *cause* of the arrow of time because *both* are emergent behavior, since neither entropy nor any other form of irreversibility exist in the microscopic laws.

I think a better way to put it is that the second law of thermodynamics is the only *evidence* we have of the arrow of time; it is the only macroscopic phenomenon that appears irreversible, since I'm pretty sure that any other irreversible process could be shown to be due to net increase of entropy.

The real answer to his question is "no". The cause of the arrow of time is currently not resolved, and the two most popular current theories as to the real answer are: a) the laws are fully symmetric, but the initial conditions are not (since the beginning of time has the big bang, and the end of time does not), or b) it is due to a phenomenon called einselection (http://en.wikipedia.org/wiki/Einselection), which is basically a kind of evolution that selects among the superposition of possible quantum outcomes, favoring the eigenstates of particle interactions, which are the subset of the possibilities that result in classical (entropic) physics.

Personally, I think einselection is a beautiful idea. In conjunction with the Many Worlds Interpretation, an increasing popular interpretation of quantum mechanics, it produces a model in which the actual physics is indeed still completely (CP)T reversible. As far as I understand it, the way it would look would be like an infinite number of microscopic futures branching off from each quantum event, including both futures where entropy increases and decreases, but where the futures where entropy increases are just a lot more likely, and tend to look very similar, so that what we perceive as classical behavior is simply the most likely, convergent path.

This whole article is a bit confusing about time reversibility, since for many years when people talk about time reversibility, they are normally talking about CPT symmetry, which is still perfectly unbroken as far as I know, since it is so well known as to be assumed that when you reverse time you just have to reverse parity and charge as well, and then it all still works perfectly. Nothing in this result changes that, it has simply been confirmed that CP reversal must go along with T reversal; you can't just reverse time by itself.

Was this a retroactive analysis of data originally collected for other purposes? If not, why didn't they publish several years earlier at the 5 sigma level?

I think that the results were determined by building the database first and then data-mining the database. It might have been that the data wasn't all in one place or database to begin with. Another reason might have been that the particular SQL query was not previously run on the data-sets. It does seem to be an extremely arcane thing to look for. (What do I know about it? nada.)

I suspect that a lot of physics PhD's are accomplished by deep-diving into old data base files looking for interesting things nobody else was looking for. Not everybody in grad school can get funding for something 'new and shiny' for their dissertation.

An analogy might be similar to all those comets found in the SOL data base of pictures. Nobody knows they exist until somebody goes and looks at the pictures.

Another crowd-sourced project is looking for new craters on the Moon. They have to do an A/B picture comparison to see a new crater pop-up. Good way to ruin your eyes.

The Second Suggestion of Thermodynamics states the entropy will most probably increase, not the it must. That means there are cases where you could spontaneously go back in time as everyone else moves forward. In other words, the Second Suggestion does not give the overwhelming dominant arrow of time we experience.

Actually, I think you have the cart before the horse. The $64k question, which is the one he is really asking, is how does the irreversible macroscopic arrow of time emerge from the completely reversible microscopic laws, which is still an open question. The second law of thermodynamics (net entropy does not decrease) cannot be the *cause* of the arrow of time because *both* are emergent behavior, since neither entropy nor any other form of irreversibility exist in the microscopic laws.

I completely disagree, in the sense that we know exactly how entropy emerges from microscopic laws, and have since the 19th century since you don't even need to know what the microscopic laws are to know where entropy comes from.

Macrostates with large numbers of microstates are more likely by a simple counting argument; therefore entropy tends to increase because systems at later times tend to be in more likely macrostates, which have more microstates. The whole idea of an emergent property is that it doesn't appear when you consider an independent particle, only a large collection. Entropy is a law of statistics that does not depend upon any of the structure of the microscopic laws: it's a universal, so long as each microstate has the same probability as any other in your microscopic model, entropy tends to increase with increasing time. Period.

Were I a computer program set up to determine whether a Universe has an arrow of time or not, I would take as input a Universe's microscopic laws. I would check that there is no intrinsic bias to certain microstates (e.g., the Universe does not prefer HHT over three coin flips to THH--bizarre Universe that would be). If that check returned true, I would return true. If not, I would have to be more careful, but certainly our Universe passes the first check.

If you want your Universe to evolve, its entropy will tend to go up in the forwards-time direction and down in the backwards-time direction. It simply cannot evolve without satisfying this requirement, and so will necessarily have an arrow of time from the outset, barring it starting in a state of maximal entropy.

The Second Suggestion of Thermodynamics states the entropy will most probably increase, not the it must. That means there are cases where you could spontaneously go back in time as everyone else moves forward. In other words, the Second Suggestion does not give the overwhelming dominant arrow of time we experience.

It's also quantum-mechanically possible for a rock to fall up temporarily in a gravitational field. That does not mean there isn't an overriding arrow of gravity generally pushing everything down, so that any video with a rock falling up in a gravitational field is backward in time with very high probability. The probabilities are also similar: your system goes backwards in time (in the sense that entropy decreases) with probability on the same negligible scale as rocks fall up, and I'm sure you'd say that that qualifies as a "strong" arrow.

The Second Suggestion of Thermodynamics states the entropy will most probably increase, not the it must. That means there are cases where you could spontaneously go back in time as everyone else moves forward. In other words, the Second Suggestion does not give the overwhelming dominant arrow of time we experience.

I always thought that entropy will increase to point where it cannot increase anymore and whole universe will be uniform after extremely long amount of time.

So is it these tiny CPT violations that work as a ratchet to create the time's arrow we perceive in the macroscopic Universe?

Entropy would do that whether CP (or equivalently, T) was violated or not, in a probabilistic sense and as an emergent effect. Meaning, the arrow of time would be discernible with some probability in collections of particles. T violation is interesting because it points to a quantum mechanical foundation for an arrow of time, distinct from the statistical one, that is not an emergent effect.

We've known this was implied by CPT since the discovery of CP violation decades ago. This direct detection of T violation is interesting only for directly confirming CPT is invariant, at least in this system. Finding otherwise would be the death of not just the Standard Model, but Quantum Field Theory in general, which would make any discrepancy in the Higgs data seem mundane by comparison.

AreWeThereYeti beat me to clarifying the question, which is whether increasing entropy is a consequence of CPT asymmetries. Nondecreasing entropy is itself a T asymmetry with no fundamental explanation of which I'm aware (not that I'm a physicist).

Edit: Saw the post above about microstates, but the fact that they are preferred in forward time more than they are in reverse time is still a T asymmetry.

"For comparison, a positive detection of the Higgs boson last summer was announced at the 5 sigma level."

Just for the record. The Higgs boson still hasn't been formally detected. What they have found so far is a boson of mass between 125 and 127 GeV with a behavior consistent with a Higgs boson. But it's going to take them years to complete the search and prove that it is the Higgs boson that comes from the Standard Model of particle physics.

Was this a retroactive analysis of data originally collected for other purposes? If not, why didn't they publish several years earlier at the 5 sigma level?

They probably did but didn't make a thing of it. The SLAC team likes to settle things once and for all with overwhelming evidence. When they release discoveries, the matter is settled end of discussion no questions taken.

"Unlike our daily experience, the world of elementary particle physics is mostly symmetrical in time. Run the clock backward on your day and it won't work; run the clock backward on a process in particle physics and things are just fine."

The Second Suggestion of Thermodynamics states the entropy will most probably increase, not the it must. That means there are cases where you could spontaneously go back in time as everyone else moves forward. In other words, the Second Suggestion does not give the overwhelming dominant arrow of time we experience.

No. A future where entropy decreases != time reversal. There are *many* valid futures with lower entropy. The past is not one of them. To reverse time, you also have to invert all charges and take a mirror image of the universe; neither of those can happen spontaneously or at all. All of the possible futures with lower entropy are*different* than such a past because of that, because those futures *don't* have reversed charge and parity, which would violate conservation of charge, momentum, and just about every other conservation law.

Example: You take a sealed container with a vacuum inside, and release a compressed gas in one corner of the container, and watch it disperse: entropy increases. Now, there are many, many valid futures (according to the microscopic laws) where the particles all come back together again, lowering entropy, leaving the rest of the box in a vacuum again. But there are at least two difference between those unlikely futures and the past: first, it is extremely likely that the particles would come back together in a different configuration or location (subject to conservation laws) than they started off with,because there are far more of those outcomes than the outcome where they all go back to the same place. Just having a lower entropy doesn't mean they all end up in the same corner they started off with, in the same positions and with the same particle velocities, momenta, etc. Secondly, as I said above, even if they all ended up again in the same spot, they would not have reversed charges or parity.

It is easy to confuse (CP)T symmetry for the possibility of spontaneous time reversal, or the possibility of actually implementing time reversal as a process, but they aren't the same thing at all. Given a simplified ideal model that ignores C and P makes it clear. Suppose you take a system consisting of a few independent particles moving through space relative to one another. T symmetry says: if you take the velocity of every particle and negate it (i.e. freeze the state, and take every particle and start it going in the *exact* opposite direction), they theoretically will retrace their paths, reversing time. But it is not *possible* for that to occur for the universe as a whole, because actually reversing the paths would involve impossible operations like violation of conservation of angular momentum, etc. It is just a statement about the symmetry of the laws under that conceptual but impossible transformation, not about valid future states.

The Second Suggestion of Thermodynamics states the entropy will most probably increase, not the it must. That means there are cases where you could spontaneously go back in time as everyone else moves forward. In other words, the Second Suggestion does not give the overwhelming dominant arrow of time we experience.

It's also quantum-mechanically possible for a rock to fall up temporarily in a gravitational field. That does not mean there isn't an overriding arrow of gravity generally pushing everything down, so that any video with a rock falling up in a gravitational field is backward in time with very high probability. The probabilities are also similar: your system goes backwards in time (in the sense that entropy decreases) with probability on the same negligible scale as rocks fall up, and I'm sure you'd say that that qualifies as a "strong" arrow.

But entropy is decreasing, see below.

dizdizzie wrote:

I always thought that entropy will increase to point where it cannot increase anymore and whole universe will be uniform after extremely long amount of time.

Entropy is decreasing with time. If it weren't, the universe won't be so cold and life wouldn't be possible. Life here on Earth exists because the Sun is hot and space is cold. Life is a heat engine that fuels its processes by the difference between the heat of the Sun and the cold of space. But how can entropy decrease when the Second Suggestion says it must increase? The universe is expanding faster than entropy is increasing. The entropy per unit volume is decreasing and life exists.

I completely disagree, in the sense that we know exactly how entropy emerges from microscopic laws, and have since the 19th century since you don't even need to know what the microscopic laws are to know where entropy comes from.

Macrostates with large numbers of microstates are more likely by a simple counting argument; therefore entropy tends to increase because systems at later times tend to be in more likely macrostates, which have more microstates. The whole idea of an emergent property is that it doesn't appear when you consider an independent particle, only a large collection. Entropy is a law of statistics that does not depend upon any of the structure of the microscopic laws: it's a universal, so long as each microstate has the same probability as any other in your microscopic model, entropy tends to increase with increasing time. Period.

Were I a computer program set up to determine whether a Universe has an arrow of time or not, I would take as input a Universe's microscopic laws. I would check that there is no intrinsic bias to certain microstates (e.g., the Universe does not prefer HHT over three coin flips to THH--bizarre Universe that would be). If that check returned true, I would return true. If not, I would have to be more careful, but certainly our Universe passes the first check.

If you want your Universe to evolve, its entropy will tend to go up in the forwards-time direction and down in the backwards-time direction. It simply cannot evolve without satisfying this requirement, and so will necessarily have an arrow of time from the outset, barring it starting in a state of maximal entropy.

No, it's not true that this has been settled since the 19th century. I assume you are talking about Boltzmann's H-theorem, but that did *not* settle the problem. The H-theorem was criticized in Loschmidt's paradox which showed that "one of the key assumptions in Boltzmann's theorem was flawed, namely that of molecular chaos, that all the particle velocities were completely uncorrelated." http://en.wikipedia.org/wiki/Loschmidt%27s_paradox .

A more recent attempt to derive it, the Fluctuation Theorem, also hasn't been accepted as unproblematic. "However, the fluctuation theorem assumes that the system is initially in a non-equilibrium state, so it can be argued that the theorem only verifies the time-asymmetry of the second law of thermodynamics based on an a priori assumption of time-asymmetric boundary conditions. [...] Thus we still have no explanation for the arrow of time that is defined by the observation that the fluctuation theorem gives correct predictions in the forward direction but not the backward direction, so the fundamental paradox remains unsolved." [Ibid]

LawofEntropy and AreWeThereYeti,Your discourse is fascinating thank you for that.I can't however get it out of my head when reading your posts the voices of Sheldon and Leonard....I really need to stop watching that show....I do however have a question, without us to observe and theorize on the nature of the "time arrow",does it really exist?

I know kind of a tree falls in the forest thing.. but i was having an argument the other day where a friend said that time is just a human construct and I was aurguing that time exists with or without us there to observe it. i.e. being able to see far into the past at distant galaxies.

LawofEntropy and AreWeThereYeti,Your discourse is fascinating thank you for that.I can't however get it out of my head when reading your posts the voices of Sheldon and Leonard....I really need to stop watching that show....I do however have a question, without us to observe and theorize on the nature of the "time arrow",does it really exist?

I know kind of a tree falls in the forest thing.. but i was having an argument the other day where a friend said that time is just a human construct and I was aurguing that time exists with or without us there to observe it. i.e. being able to see far into the past at distant galaxies.

Time in the sense of cause before effect exists. You should note that physicists have stopped using "observe" and are now using "measure", for two reasons. First, observed means you have little effect on it, which is not the case with quanta. Second, observing is something a person does but measuring is something a device does. People are not special, just another measuring device.

Time is the sense that it has a dimension does not exist. The dimensionality of time is borrowed from the cyclic movement of a clock, usually an imaginary one.

LawofEntropy and AreWeThereYeti,Your discourse is fascinating thank you for that.I can't however get it out of my head when reading your posts the voices of Sheldon and Leonard....I really need to stop watching that show....I do however have a question, without us to observe and theorize on the nature of the "time arrow",does it really exist?

I know kind of a tree falls in the forest thing.. but i was having an argument the other day where a friend said that time is just a human construct and I was arguing that time exists with or without us there to observe it. i.e. being able to see far into the past at distant galaxies.

Kind of off topic, but: There was a whole pseudo-mystical movement in the 20th century spawned by misinterpretation of quantum mechanics, that thought that the collapse of waves to particles upon observation meant that a conscious observer was necessary, and therefore that conscious observers were somehow special. Many people leapt to the conclusion that it provided a negative answer to the old question about whether a tree falls in the forest if no one is there to observe it, and that conscious observers therefore "create" reality.

But that is all derived from a very poor understanding of what "observation" means. It has been become clear since then that "observation" (normally just called "measurement") is just a kind of correlation between any two systems, that is created whenever they interact in such a way as to transfer information about one to the other. In other words, a wave becomes a particle *relative* some other system, when that other system gains information about some particle property about it, like position, spin, etc. I'm hand waving away subtle and tricky issues about "premeasurement" here, but they don't affect the point.

The same is true for time's arrow. Its existence has to do with ability of some system to do experiments to gain information about the direction of times arrow. The system doesn't have to be conscious or human. It just has to encode the information somehow.

[edit: and the system can be just about any macroscopic system. The tree falls in the forest because there are an uncountable multitude of subtle interactions between the falling tree and its surroundings, and those surroundings "observe" the tree, and the tree falls for you when you "observe" those surroundings, directly or indirectly.]

If I had used measurement in place of observe the point would still be the same?sorry about the off topic question....I was alittle confused about this statement.

It is easy to confuse (CP)T symmetry for the possibility of spontaneous time reversal, or the possibility of actually implementing time reversal as a process, but they aren't the same thing at all. Given a simplified ideal model that ignores C and P makes it clear. Suppose you take a system consisting of a few independent particles moving through space relative to one another. T symmetry says: if you take the velocity of every particle and negate it (i.e. freeze the state, and take every particle and start it going in the *exact* opposite direction), they theoretically will retrace their paths, reversing time. But it is not *possible* for that to occur for the universe as a whole, because actually reversing the paths would involve impossible operations like violation of conservation of angular momentum, etc. It is just a statement about the symmetry of the laws under that conceptual but impossible transformation, not about valid future states.

I suppose you could substitute time for tree in this.[edit: and the system can be just about any macroscopic system. The tree falls in the forest because there are an uncountable multitude of subtle interactions between the falling tree and its surroundings, and those surroundings "observe" the tree, and the tree falls for you when you "observe" those surroundings, directly or indirectly.]Thanks for the clarifcation

I know kind of a tree falls in the forest thing.. but i was having an argument the other day where a friend said that time is just a human construct and I was aurguing that time exists with or without us there to observe it. i.e. being able to see far into the past at distant galaxies.

Yes, it does. The CMB depicts a Universe with very low entropy, nearly perfectly homogenous. The Universe today has much, much higher entropy, although it is still nearly homogenous. That alone is enough to establish that, over 13.78 billion years of no humans, the Universe has been subject to the same arrow of time as drives it today.

The CMB depicts a Universe with very low entropy, nearly perfectly homogenous.

Nearly perfect homogeneous implies high entropy. The universe started with very high entropy but it expanded faster than entropy increased, leaving us today with a universe with very low entropy per unit volume.

I'm curious if the problem lies with our perception and understanding of time? The concept of different time directions is intriguing and I hope this leads to furthur research.

I apologize as this is off topic, but I've been stuck on the idea that we do not properly understand the phenomenon of time for a while.. mainly because time zero does not make sense (the best explanation I've gotten is the strength of gravity was so great it effectively paused time, but that still doesn't deliver a solution to the before/where). Hopefully someone here is smarter than I and maybe has at least a hypothesis that I can live with or maybe somewhere in my lifetime someone will come up with one as my brain is not large enough to do so.

As for the time travel thoughts, I've always felt forward travel could be possible, backwards impossible (but viewable) based on our current concepts. Though I'm not a physicist and don't pretend to be, so take that with a grain of salt.

thanks for humoring me.

In my opinion, you've got it just about right. I have yet to wrap my head around time zero.